This Add-on is useful if you have a lot of Google Calendars.  Google Calendar gives you an option to 'Display Only This Calendar', which unselects all other calendars.  But what about when you want to select all of your calendars back again?

Once installed, choose Display All Calendars' in the Add-ons menu. Dialog boxes will show you which calendars are being selected. The final message tells you to refresh your Google Calendar. After doing so, you will see that all of your calendars have been selected.

If you want a list of all of your calendars, choose 'List All Calendars.' This will list out all calendars in your spreadsheet stacked vertically and starting where your cursor is placed.  Warning- it will overwrite any data in those cells.